Heat treatment of chrysotile asbestos fibres

ABSTRACT

Asbestos fibres having excellent properties in resin reinforcement are produced by controlled heat treatment of a coarse fraction of chrysotile asbestos fibres.

FIELD OF INVENTION

This invention relates to the production of novel asbestos fibres fromconventional soft chrysotile asbestos.

BACKGROUND TO THE INVENTION

Asbestos is used as a reinforcing fibre in fibre-filled plasticmaterials. Two important properties of the resulting composite materialare the impact strength and the flexural strength. When most types ofchrysotile asbestos are used as reinforcing fibres, the compositematerial has generally relatively good flexural strength and stiffnessbut poor impact strength. In many instances, it is thereforeadvantageous to improve the impact strength.

Some naturally occurring, very harsh chrysotile asbestos fibre havingparticularly good flexural strength and stiffness properties as areinforcing fibre in fibre-filled resins is known to be relatively long,very straight and smooth-surfaced. Available quantities of this fibreare limited and a comparable asbestos fibre has not heretofore beenproduced synthetically.

SUMMARY OF INVENTION

In accordance with the present invention, chrysotile asbestos fibres areheat treated to obtain an improvement in impact strength properties fora coarse fraction thereof when used in resin reinforcement withoutincurring any strength loss through loss of water of crystallizationother than a very minor loss. The coarse fraction may be initiallyseparated from the chrysotile asbestos fibres or may be separated fromheat treated fibres.

The heat treatment is effected in an ambient atmosphere having asubstantially uniform temperature of about 400° to about 600° C. for atime from about 1 to about 5 hours sufficient to increase the impactstrength of the coarse fraction of fibres in resin reinforcement andinsufficient to result in a significant degree of dehydroxylation of thefibres, which would impair the impact strength properties. The heatingmay be effected in any convenient apparatus, such as, a large scalemuffle furnace.

The heating of the fibres is controlled by a temperature-timerelationship to avoid loss of water of crystallization and the maximumwater loss (dehydroxylation) which can be tolerated is about 2% byweight of the total water of crystallization, the latter value being the"significant degree of dehydroxylation" mentioned above. It is preferredto achieve the increased impact strength properties without anydehydroxylation.

GENERAL DESCRIPTION OF INVENTION

The asbestos fibres may be treated by the procedure of the invention ina bulk form but it is more convenient and preferred to treat the fibresin a pressure packed form. Pressure packing decreases considerably thevolume of the fibres to be treated and permits commercial operation,since large volumes of fibres can be simultaneously heat treated.

When in the pressure packed form, it takes some time for the heat topermeate the block, so that fibres in outer portions of the block may beheat treated at the ambient atmosphere temperature for longer periodsthan those towards the centre of the block. This heating pattern resultsin an overall improvement in impact strength properties in resinreinforcement for the coarse fibres in the block without strength loss,although the individual properties of the fibres in the block may vary.

Pressure packing, when effected, is preferably carried out to provide adensity in the pressure pack of about 25 to about 50 lb./cu.ft. (200 to800 Kg/M³). The pressure pack typically is formed from a 100 lb. (50 Kg)lot of fibres and may be dimensioned 24"×16"×9".

In a preferred embodiment of the invention, the chrysotile fibres areheated in an ambient atmosphere having a temperature of about 500° toabout 550° C. for about 2 to 3 hours, usually in a pressure packed formhaving a density of about 25 to about 50 lb./cu.ft.

Two alternative procedures are possible within the scope of theinvention to produce the novel fibres having high impact strengthproperties. In the first of these alternatives, the chrysotile asbestosfibres are heat treated and, thereafter, the heat treated fibres areclassified and the coarse fraction is collected.

In one embodiment of this procedure, the heat treated fibres are openedto a limited extent by any convenient procedure, such as, by using a fanopener to an average surface area of about 4000 to about 8000 sq.cm/g.The fibres then are classified to separate the coarse fraction. Theclassification may be effected using an air classifier, particularly aCentri-Sonic classifier.

The coarse fibre fraction obtained in this embodiment constitutes about10 to about 35% of the total opened fibres, with the quantity varyingwith the temperture of heat treatment. The ratio of coarse fraction tofines fraction produced also may be varied by controlling the speed ofrotation of the classifier and the rate of air flow therethrough.

The fines fraction from the classification may be discarded or usefullyused for asbestos cement reinforcement with improved filtrationproperties, in accordance with the disclosure of our U.S. applicationSer. No. 824,929 filed Aug. 15, 1977.

In the other alternative, the chrysotile asbestos fibres first areclassified to separate a coarse fraction which then is heat treated. Thefines fraction may be discarded or separately heat treated in accordancewith our aforementioned U.S. application.

While the procedure of the invention has wide applicability to a varietyof grades of chrysotile asbestos fibres, it has particular utility withrespect to those chrysotile asbestos fibres classified as Group 4 andGroup 5 by Quebec Asbestos Mining Association.

The heating process carried out in accordance with this inventionchanges the physical characteristics of the coarse fraction of asbestosfibres. The heat treated coarse fraction resulting from the process ofthe invention is in the form of long, very straight and smooth-surfacedhard fibres which are longitudinally alignable for thermosetting resinreinforcement and readily dispersed for thermoplastic resinreinforcement.

The fibres exhibit a high impact strength in fibre-filled resins whichis greater than the natural very harsh fibres, although they exhibitlower flexural strength than the natural very harsh fibres. The fibresproduced in this invention generally have diameters of about 0.1 mm toabout 2 mm and lengths up to about 8 mm.

The coarse fibres provided following the procedure of the invention maybe opened further, typically to an average surface area of about 3000cm² per gram. The further opened fibres have both the appearance andreinforcing properties of the natural very harsh fibre, including theflexural strength properties.

EXAMPLE

Chrysotile asbestos fibres (Group 4) were formed into a pressure-packedblock and heated at 500° C. for 2 hours in a muffle furnace. Followingthe heat treatment and cooling of the block, the fibres were opened toan average surface area of about 7000 cm² /g using a fan opener.

The opened fibres were fed to a Centri-Sonic classifier operating underconditions which will produce 10 to 15% rejects. The rejects wererelatively coarse and very straight fibres. The reinforcingcharacteristics of this fraction in high density polyethylene weretested and compared with those of 1/2 inch glass fibres, anotherchrysotile material (Calidria RG600) widely used as a reinforcingmaterial, the very harsh fibre and the rejects fraction opened to 3000cm² /g.

In each case, the resin and fibres (30% by weight of mixture) werecompounded on a two-roll mill for 2 minutes at 275° F., cooled andbroken into pieces approximately 1"×1"×1/2". The pieces were compressionmolded at 275° F. and 1000 psi.

The resulting materials were subjected to various tests and the resultsare reproduced in the following Table I:

                  TABLE I                                                         ______________________________________                                                  Izod Impact                                                                            Flexural Flexural                                                    Strength Strength Modulus                                                     (ft.lb./inch)                                                                          (psi)    (psi)    Colour                                   ______________________________________                                        Very Harsh Fibre                                                                          0.46       8000     710,000                                                                              Black                                  Calidria RG 600                                                                           0.66       7420     624,000                                                                              Dark                                                                          Green                                  Glass Fibres                                                                              1.26       5990     537,000                                                                              White                                  Classified Coarse                                                                         0.99       6900     580,000                                                                              Light                                  Fraction (unopened)                    Brown                                  Classified Coarse                                                             Fraction Opened to                     Light                                  3000 cm.sup.2 /g                                                                          0.45       7600     710,000                                                                              Yellow                                 ______________________________________                                    

The results of the above Table I show the high impact strengthproperties of the coarse fraction and the very close comparison of theproperties of the opened fraction and the very harsh fibre.

The fines fraction from the classifier was tested for its properties inasbestos-cement. These properties were compared with those of untreatedfibres which had been otherwise processed to produce their optimumproperties. The results are reproduced in the following Table II:

                  TABLE II                                                        ______________________________________                                                      Classified Fines                                                Property      Fraction    Untreated Fibres                                    ______________________________________                                        Filtration rate                                                               (ml/sec)      22.8        13.1                                                Water retention (%)                                                                         16.2        20.4                                                Plaque density (g/cm.sup.3)                                                                 1.41        1.60                                                Modulus of rupture                                                            adjusted for density                                                          (kg.cm.sup.2) 516         527                                                 ______________________________________                                    

The results of the above Table II demonstrate that the heat treated,opened and fine classified fibres exhibit substantially no decrease inmodulus of rupture and an increase in asbestos-cement filtration rate ofabout 75%.

SUMMARY

The present invention, therefore, provides a procedure for forming novelasbestos fibres from chrysotile asbestos fibres. Modifications arepossible within the scope of the invention.

What we claim is:
 1. A method for the formation from Group 4 or Group 5chrysotile asbestos fibres of long, very straight, smooth-surfaced, hardasbestos fibres having enhanced impact strength in resin reinforcement,which comprises:initially separating a coarse fraction from saidchrysotile asbestos fibres and heat treating said coarse fraction toform said hard asbestos fibres; said heat treatment being effected byheating in an ambient atmosphere having a substantially uniformtemperature of about 400° to about 600° C. for a time from about 1 toabout 5 hours sufficient to increase the impact strength of the coarsefraction of fibres in resin reinforcement and insufficient to result ina loss of water of crystallization of the fibres of greater than about2% by weight of the total weight of water of crystallization.
 2. Themethod of claim 1 wherein said heat treatment is effected to achieve amaximum increase in said impact strength properties without any loss ofwater of crystallization from the fibres.
 3. The method of claim 1wherein said heat treatment is effected in an ambient atmosphere at atemperature of about 500° to about 550° C. for about 2 to 3 hours. 4.The method of claim 1 wherein said fibres are in a pressure packed formduring said heating step.
 5. The method of claim 4 wherein said fibresare pressure packed to a density of about 25 to about 50 lb./cu. ft. 6.The method of claim 1 including opening said coarse fraction to anaverage surface area of about 3000 cm² /g.